1. Technical Field of the Invention
The present invention relates to alumina-, cerium oxide- and zirconium oxide-based compositions exhibiting high reducibility, to a process for the preparation thereof and to the use of same as catalysts, in particular for the treatment or conversion of exhaust gases emanating from internal combustion engines.
2. Description of the Prior Art
It is known to this art that catalysts may be in the form of a carrier, such as a heat-resistant ceramic unitary shaped article or monolith, or a metallic substrate, coated with a coating typically comprising a material that provides a porous layer, e.g., alumina, and catalytically active elements such as precious metals and other elements, for example rare earth oxides, in particular cerium oxide or zirconium oxide, which may impart their own catalytic function and/or a carrier function for precious metals. This coating is typically referred to as a "washcoat."
The alumina/oxides combination, or the compound based on these elements, must provide a number of properties to be effective as a catalyst. One of these properties is reducibility, which signifies, both here and in the description to follow, the capacity to become reduced under a reducing atmosphere and to become reoxidized under an oxidizing atmosphere. Another such property is stability under temperature, with "stability" connoting the stability of the specific surface area of the composition. Indeed, the composition must retain a sufficiently large surface area after having been exposed to high temperatures. "Stability" also connotes the capacity to store hydrogen or oxygen. As regards the specific surface area, this capacity must not vary appreciably after several cycles of exposure to high temperature.